Electricity Bill & Energy Cost Calculator

The arithmetic

Units (kWh) = watts × hours × quantity / 1000
Cost        = units × rate + fixed charge + duty

A unit is a kilowatt-hour: one kilowatt drawn for one hour. A 1,500 W air conditioner running 8 hours a day uses 12 units a day and 360 a month — which on most tariffs is the difference between a modest bill and an alarming one.

Currencies

Pick your currency in the tariff panel and every rate, charge and result switches with it, formatted the way your region writes numbers — Indian lakhs, European decimal commas, and so on. The choice is remembered, so it carries across to the solar sizing calculator too.

Changing currency also re-seeds the rates with typical local figures, and switches between slab and flat-rate billing depending on which is normal there. Those figures are rough starting points that go out of date — anything you type over is kept when you change currency again, because your own numbers are the point of the exercise.

Why a flat rate gives the wrong answer

Indian, Pakistani, Bangladeshi, Sri Lankan and South African domestic tariffs are typically telescopic: each slab's rate applies only to the units that fall inside it, not to the whole bill. If the first 100 units are ₹4.50 and the next 100 are ₹6.00, then 150 units costs (100 × 4.50) + (50 × 6.00) = ₹750, not 150 × 6.00.

The consequence matters more than the arithmetic: the last units you consume are the expensive ones. On a 400-unit bill, an extra 50 units are charged at the top rate, not the average. So a new appliance always costs more to run than your average rate suggests — and anything you switch off always saves more.

This is the same reason solar looks better than a naive calculation suggests: the units it displaces come off the top slab first. It is worked through in how many solar panels for 1000 units.

The slab boundaries and rates in the calculator are editable and the defaults are illustrative only. Every state sets its own, and they change with each tariff order — copy the ones printed on your own bill. Some tariffs also switch to a different structure entirely above a threshold, so a very high consumption month may not follow the same table.

The charges that are not per unit

The fixed charge is why the effective rate shown by this calculator is always higher than the slab rate, and why it falls as consumption rises — the fixed part is spread over more units.

On a commercial connection the demand charge is billed on kVA, not kW, which is where power factor turns into money. That is a separate calculation — see kW, kVA and kVAr and what the power factor penalty costs.

Nameplate watts are not running watts

The figure on the label is what the appliance draws at full output. Very little runs that way:

• Air conditioners cycle. A 1.5-ton unit rated 1,500 W might average 900 W over a night once the room is cool — and an inverter AC modulates continuously rather than cycling, so its average is lower still.
• Refrigerators run a compressor perhaps a third of the time. Use the annual kWh from the BEE star label rather than the nameplate watts.
• Motors draw according to the load on the shaft, not their rating. A pump against a throttled valve draws less than its nameplate — and wastes most of what it does draw.
• Standby is small and constant. A few watts × 24 × 365 across a dozen devices is a real number.

For anything that cycles, the honest input is the average power over the running period, not the nameplate. A clamp meter or a plug-in energy monitor settles it in an afternoon.

Is it worth replacing?

The comparison panel prices the replacement at the same tariff and reports the payback. Two things to keep in mind when reading it.

The saving is valued at your marginal rate — the top slab — which is correct, because those are the units that disappear. This is why efficiency upgrades look better on a large bill than on a small one, for exactly the same appliance.

And a payback figure ignores what the old appliance would have cost you anyway. If a fifteen-year-old refrigerator is near the end of its life, the right comparison is not “new versus old” but “efficient new versus cheap new” — which usually pays back in a year or two rather than five.

Where the money usually is

Running this calculator across a household or a small plant tends to produce the same short list:

1. Anything with a heating element. Geysers, irons, kettles, ovens. Full power, no efficiency to improve — only run time.
2. Air conditioning. High power and long hours. One degree on the thermostat is worth more than most equipment changes.
3. Anything running when nobody is there. Base load is always the cheapest saving, and it is invisible without measuring — see turning meter data into savings.
4. Old motors and pumps. An IE1 motor replaced with IE3 saves a few per cent, and running it only when needed saves far more.